LEDs are not only compact and highly efficient as compared to existing light sources that utilize electric discharge or radiation, but recently also are gaining an increased luminous flux. LEDs thus are expected to replace the existing light sources soon. Furthermore, because LEDs are more compact than light sources that utilize electric discharge or radiation, they have the following advantages: they have a wider range of applications, are easier to handle and are available in various designs as required. Accordingly, LEDs are considered as value-added light sources.
Light sources for lighting applications are required to have, as one of the characteristics, color uniformity, that is, no light color difference depending on the emitting direction of light emitted. In a light-emitting device that includes a wavelength converting material such as a fluorescent material, primary light emitted from the light-emitting element and secondary light obtained through absorption and conversion of the primary light by the wavelength converting material are combined, and light such as white light is emitted. In this case, the resulting color differs depending on the mixing ratio of the primary light and the secondary light. This mixing ratio may differ depending on the emitting direction, which causes a light color difference depending on the emitting direction, and a light color difference across the light-emitting surface (light extraction surface) of the wavelength converting portion.
The following methods have been proposed for suppressing such light color difference.
First, a method has been proposed in which a wavelength converting portion (a resin layer that includes fluorescent material particles) is formed on an LED chip serving as a light-emitting element uniformly over the shape of the LED chip (see, for example, Patent Document 1). With this configuration, the distance over which primary light passes through the wavelength converting portion can be made uniform, as compared to a configuration in which the wavelength converting portion is formed on the LED chip by dripping or the like, and it is therefore possible to suppress the light color difference in the light extraction surface of the wavelength converting portion.
Light sources for lighting applications that provide a high luminous flux are required to emit light of uniform color to the irradiation surface. Accordingly, a further reduction in the light color difference across the light extraction surface of the wavelength converting portion is necessary.
Inorganic fluorescent material particles, which usually are used as a wavelength converting material, are known to have a reflection function in addition to the wavelength converting function, so that a scattering action is effected, and as a result, primary light and secondary light are scattered, which can suppress the light color difference (see, for example, Patent Document 2).
Another method has been proposed in which the vicinity of the center of a wavelength converting portion that includes a wavelength converting material, that is, the area directly above a light-emitting element, is made thick and the periphery of the wavelength converting portion is made thin (see, for example, Patent Document 3). Patent Document 3 states that “by reducing a light path length difference across the wavelength converting portion between primary light that travels in the vertically upward direction from the light-emitting element and primary light that travels in an obliquely upward direction from the light-emitting element, the ratio between the primary light and the secondary light becomes substantially equal, and thus color nonuniformity is reduced”. This is based on the model that “if the distribution of the wavelength converting material in the wavelength converting portion is uniform, the light path length has a correlation with the frequency at which primary light hits the wavelength converting material, and the ratio between the primary light and the secondary light is determined based on this frequency”.
Another method has been proposed in which the vicinity of the center of a wavelength converting portion that includes a wavelength converting material, that is, the area directly above a light-emitting element, is caused to have a higher concentration of the wavelength converting material than that of the periphery of the wavelength converting portion (see, for example, Patent Document 4). This is based on the model that “if the thickness of the wavelength converting portion is uniform, by reducing a difference in frequency at which light hits the wavelength converting material of the wavelength converting portion between primary light that travels in the vertically upward direction from the light-emitting element and primary light that travels in an obliquely upward direction from the light-emitting element, the ratio between the primary light and the secondary light becomes substantially equal, and thus color nonuniformity is reduced”.    Patent Citation 1: U.S. Pat. No. 6,468,821B2    Patent Citation 2: JP H7-99345A    Patent Citation 3: Japanese Patent No. 3065263 (paragraph [0016], FIG. 2)    Patent Citation 4: JP 2005-166733A (paragraph [0035], FIG. 8)